The present invention relates generally to instrumentation for characterization of molecules based at least on their structures and mass-to-charge ratios as gas-phase ions, and more specifically to such instrumentation which provides for rapid and sensitive analysis of composition, sequence, and/or structural information relating to organic molecules, including biomolecules, and inorganic molecules.
Biological molecules, such as DNA, RNA, proteins, carbohydrates and glycoconjugates, are comprised of repeating subunits typically referred to as residues. The sequence of such residues ultimately defines the structure and function of the biomolecule and determines how it will interact with other molecules.
A central part of almost all conventional sequencing strategies is the analysis of complex sets of sequence-related molecular fragments by chromatography or by polyacrylamide gel electrophoresis (PAGE). PAGE-based automated sequencing instruments currently exist and typically require a number of fluorescent dyes to be incorporated into the base-specifically terminated biomolecule product, which is then processed through the polyacrylamide gel. The discrete-length product molecules are detected near the bottom of the gel by their emitted fluorescence following excitation by a radiation source.
Such automated instruments are typically capable of generating sequence information for biomolecules having 500 or more residues at a rate of 10-20 times faster than manual methods. However, both the manual and automated PAGE techniques suffer from several drawbacks. For example, both approaches are labor-intensive since a gel must be prepared for each sequencing run. Also, while automated PAGE systems may offer faster analysis times than a manual approach, the accuracy of such systems is limited by artifacts generated by non-uniform gel matrices and other factors. Such automated systems are generally not equipped to accurately process the effects of such artifacts, which are typically manifested as xe2x80x9csmilingxe2x80x9d compressions, faint ghost bands, and the like. Manual interpretation of such results is therefore often required which significantly increases analysis time.
Researchers have, within the past several years, recognized a need for more rapid and sensitive techniques for analyzing the structure and sequences of biomolecules. Mass spectrometry (MS) techniques, such as time-of-flight mass spectrometry (TOFMS) and Fourier Transform ion-cyclotron-resonance mass spectroscopy, are well known techniques for quickly and accurately providing ion mass information from which sequence and structural determinations can be made. As is known in the art, TOFMS systems accelerate ions, via an electric field, toward a field-free flight tube which terminates at an ion detector. In accordance with known TOFMS principles, ion flight time is a function of ion mass so that ions having less mass arrive at the detector more quickly than those having greater mass. Ion mass can thus be computed from ion flight time through the instrument. FIG. 1 demonstrates this principle for a cytochrome-c sample, having a known mass to charge ratio (m/z) of 12,360 da, and a lysozyme sample, having a known mass to charge ratio (m/z) of 14,306 da. In FIG. 1, signal peak 10, having a flight time of approximately 40.52 xcexcs corresponds to the lighter cytochrome-c sample, and signal peak 12, having a flight time of approximately 41.04 xcexcs, corresponds to the heavier lysozyme sample.
Due to the significantly decreased sample preparation and analysis times of MS techniques over the above-described PAGE technique, several MS sequencing strategies have recently been developed. Such MS sequencing techniques are generally operable to measure the change in mass of a biomolecule as residues are sequentially removed from its end. Examples of two such techniques, each involving elaborate pre-MS processing techniques, are described in U.S. Pat. No. 5,210,412 to Levis et al. and U.S. Pat. No. 5,622,824 to Kxc3x6ster.
In order to provide for the capability of determining sequence and structural information for large biomolecules, it has been recognized that MS techniques must accordingly be capable of generating large ions. Currently, at least two techniques are known for generating large ions for spectral analysis; namely electrospray ionization (ESI) and matrix assisted laser desorption ionization (MALDI). While both large ion generating techniques are readily available, known MS techniques are limited in both the quantity and quality of discernable information. Specifically, for large biomolecules, defined here as those containing at least 50 residues, mass spectra of parent and sequence related fragment ions become congested to the degree that mass (TOF) peaks overlap.
One solution to the problem of congested mass spectra is to increase the mass resolution capability of the MS instrument. Recent efforts at increasing such resolution have been successful, and complete sequence information for a 50 base pair DNA has been obtained using a Fourier Transform ion cyclotron resonance (FTICR) instrument. However, such instruments are extremely expensive, not readily available, and because of their extremely high vacuum requirements, they are generally not suitable for routinely sequencing large numbers of samples.
Another solution to the problem of congested mass spectra is to pre-separate the bulk of ions in time prior to supplying them to the ion acceleration region of the MS instrument. Mass spectrometry can then be performed sequentially on xe2x80x9cpacketsxe2x80x9d of separated ion samples, rather than simultaneously on the bulk of the generated ions. In this manner, mass spectral information provided by the MS instrument may be spread out over time in a dimension other than mass to thereby reduce the localized congestion of mass information associated with the bulk ion analysis.
One known ion separation technique which may be used to pre-separate the bulk of the ions in time prior to MS analysis is ion mobility spectrometry (IMS). As is known in the art, IMS instruments typically include a pressurized static buffer gas contained in a drift tube which defines a constant electric field from one end of the tube to the other. Gaseous ions entering the constant electric field area are accelerated thereby and experience repeated collisions with the buffer gas molecules as they travel through the drift tube. As a result of the repeated accelerations and collisions, each of the gaseous ions achieves a constant velocity through the drift tube. The ratio of ion velocity to the magnitude of the electric field defines an ion mobility, wherein the mobility of any given ion through a high pressure buffer gas is a function of the collision cross-section of the ion with the buffer gas and the charge of the ion. Generally, compact conformers, i.e. those having smaller collision cross-sectional areas, have higher mobilities, and hence higher velocities through the buffer gas, than diffuse conformers of the same mass, i.e. those having larger collision cross-sectional areas. Thus, ions having larger collision cross-sections move more slowly through the drift tube of an IMS instrument than those having smaller collision cross-sections, even though the ions having smaller collision cross-sections may have greater mass than those having higher collision cross-sections. This concept is illustrated in FIG. 2 which shows drift times through a conventional IMS instrument for three ions, each having a different mass and shape (collision cross-section). As is evident from FIG. 2, the most compact ion 14 (which appears to have the greatest mass) has the shortest drift time peak 16 of approximately 5.0 ms, the most diffuse ion 18 has the longest drift time peak 20 of approximately 7.4 ms, and the ion 22 having a collision cross-section between that of ion 14 and ion 18 (which also appears to have the least mass), has a drift time peak 24 of approximately 6.1 ms.
Referring now to FIG. 3, an ion time-of-flight spectrum 26, obtained from a known time-of-flight mass spectrometer, is shown plotted vs. ion drift time. In this figure, ions of different mass are dispersed over different times of flight in the mass spectrometer. However, due to the limited resolution of the mass spectrometer, ions are not completely separated in the spectrum, i.e. dots corresponding to different ions overlap. When compared with FIG. 6, which will be discussed more fully in the DESCRIPTION OF THE PREFERRED EMBODIMENTS section, it is evident that different ions can be better resolved by an instrument that separates ions in two dimensions, namely ion mobility and ion mass.
Guevremont et al. have recently modified an existing IMS/MS instrument to convert a quadrupole MS to a TOFMS [R. Guevremont, K. W. M. Siu, and L. Ding, PROCEEDINGS OF THE 44TH ASMS CONFERENCE, (1996), Abstract]. Ions are generated in the Guevremont et al. instrument via electrospray, and 5 ms packets are gated into the IMS instrument. The ion packets produced by the IMS instrument are passed through a small opening into an ion acceleration region of the TOFMS.
While Guevremont et al. have had some experimental success in coupling an IMS instrument to a TOFMS instrument, their resulting instrumentation and techniques have several drawbacks associated therewith. For example, since the Guevremont et al. abstract discusses using 5 ms gate pulses to admit ions into the IMS instrument, it is noted that the resultant IMS spectrum has low resolution with at least 5 ms peak widths. Secondly, because the drift tube and ion flight tube of the Guevremont et al. instrument are colinear, any spatial and temporal spread in an ion packet leaving the IMS leads directly to a spatial and temporal spread of ions in the ion acceleration region of the TOFMS. These two characteristics lead to poor mass resolution in the TOFMS. The combination of low resolution in the IMS and low resolution in the TOFMS makes this instrument incapable of resolving complex mixtures. What is therefore needed is a hybrid IMS/TOFMS instrument optimized to resolve complex mixtures. Such an instrument should ideally provide for optimization of the ion mobility spectrum as well as optimization of the mass spectrum. Moreover, such a system should provide for an optimum interface between the two instruments to thereby maximize the capabilities of the TOFMS.
The foregoing drawbacks associated with the prior art systems discussed in the BACKGROUND section are addressed by the present invention. In accordance with one aspect of the present invention, a method of separating ions in time comprises the steps of separating a bulk of ions in time as a function of a first molecular characteristic, sequentially separating in time as a function of ion mobility at least some of the ions previously separated in time as a function of a first molecular characteristic, and sequentially separating in time as a function of ion mass at least some of the ions previously separated in time as a function of ion mobility.
In accordance with another aspect of the present invention, an apparatus for separating ions in time comprises means for separating a bulk of ions in time as a function of a first molecular characteristic, an ion mobility spectrometer (IMS) having an ion inlet coupled to the means for separating a bulk of ions in time as a function of a first molecular characteristic and an ion outlet, wherein the IMS is operable to separate ions in time as a function of ion mobility. A mass spectrometer (MS) is further included and has an ion acceleration region coupled to the ion outlet of the IMS, wherein the MS is operable to separate ions in time as a function of ion mass.
In accordance with a further aspect of the present invention, a method of separating ions in time comprises the steps of separating a bulk of ions in time according to a first function of ion mobility, sequentially separating in time according to a second function of ion mobility at least some of the ions separated in time according to the first function of ion mobility, wherein the second function of ion mobility is different from the first function of mobility, and sequentially separating in time as a function of ion mass at least some of the ions separated in time according to the second function of ion mobility.
In accordance with still another aspect of the present invention, an apparatus for separating ions in time comprises a first ion mobility spectrometer (IMS1) having an ion inlet and an ion outlet, wherein the IMS1 is operable to separate ions in time according to a first function of ion mobility and a second ion mobility spectrometer (IMS2) having an ion inlet coupled to the ion outlet of the IMS1 and an ion outlet, wherein the IMS2 is operable to separate ions in time according to a second function of ion mobility different from the first function of ion mobility. A mass spectrometer is also included and has an ion acceleration region coupled to the ion outlet of the IMS2, wherein the mass spectrometer is operable to separate ions in time as a function of ion mass.
One object of the present invention is to provide instrumentation for rapid analysis.and sequencing of large biomolecules, as well as analysis of mixtures of organic and inorganic molecules.
Another object of the present invention is to provide an ion mobility and mass spectrometer for composition, sequence and structural analysis of biomolecules.
Yet another object of the present invention is to provide such an instrument operable to produce molecular information separated in time according to at least three different molecular characteristic functions.
Still another object of the present invention is to provide such an instrument wherein two of the three different molecular characteristic functions are ion mobility and ion mass/charge, and wherein the third molecular characteristic function may be ion retention time, a second different ion mobility or the like.
Still a further object of the present invention is to provide a technique for operating such an instrument in obtaining sequencing information.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiments.